1. Field of the Invention
The present invention relates to a semiconductor device of the type having bump electrodes projecting from the surface of a chip. Such type of semiconductor device is so-called as a "flip-chip type" semiconductor device.
2. Description of the Related Art
To accomplish small electronic units, the sizes of electronic parts have been decreased with advanced functions and high integration.
Semiconductor devices such as electronic parts are coated with resin. In order to further minimize the sizes of semiconductor devices, bare chips have been used. The bare chips are directly mounted on wiring boards.
FIG. 1 shows an example of a conventional bare-chip type semiconductor device. In FIG. 1, reference numeral 1 depicts a semiconductor pellet having a semiconductor substrate 2 and many semiconductor devices (not shown) formed therein. The semiconductor devices form a particular electronic circuit with internal connections in the semiconductor substrate 2. The semiconductor substrate 2 has an insulation film (not shown) formed on one main surface thereof. This insulation film has opening portions for which the semiconductor devices are connected. A base electrode 3 is disposed at each opening portion of the insulation film. The base electrode 3 is connected to a corresponding bump electrode 4.
The base electrode 3 comprises a single layer of aluminum or a laminate of a plurality of layers of metal such as titanium and gold. The base electrode 3 is connected to the semiconductor substrate 2 with a resistance. When the base electrode 3 is a laminate of layers of titanium and gold, the bump electrode 4 comprises a metal which has a good connecting characteristic. An example of the material of the bump electrode 4 is a plated layer of gold.
In FIG. 1, reference numeral 5 depicts a wiring board. A conductive pattern is formed on an insulation board 6 composed of an insulation material such as ceramics or resins. As shown in a circular window on the right side of FIG. 1, an electrode portion 7 is disposed opposite to the bump electrode 4 of the semiconductor pellet 1. When the insulation board 6 comprises a ceramic material, a conductive pattern is formed by a printing process or baking process of a silver palladium conductive paint. On the other hand, when the insulation board 6 comprises a resin, the conductive pattern is formed by an etching process of a conductive foil in a desired pattern.
The conductive pattern except for the electrode portion 7 and an external connection terminal (not shown) is coated with a resist film (not shown). The electrode portion 7 which is not coated with the resist film is coated with hard metal layer 8 comprising metal such as nickel and a thin gold film 9 which allows an electric connection to be secured, thereby to constitute a pad electrode 10.
The semiconductor pellet 1 and the wiring board 5 are disposed so that they face each other. The bump electrode 4 and the pad electrode 10 are layered. The bump electrode 4 is heated and pressed at the layered portion so as to deform plastically the bump electrode 4. Thus, since gold of the bump electrode 4 and gold of the thin film 9 of the pad electrode 10 are press-fitted, an electric connection between the bump electrode 4 and the gold thin film 9 is secured.
Reference numeral 11 depicts a resin which bonds the semiconductor pellet 1 to the wiring board 5. The resin is coated on the wiring board 5 before the above-mentioned press-fitting process is performed. Alternatively, when the electrodes 4 and 7 are press-contacted, the resin is filled in the space between the semiconductor pellet 1 and the wiring board 5. Then, in the pressuring state of the semiconductor pellet 1, the resin is heated at around 200.degree. C. for a few minutes so as to harden the resin.
When the resin 11 hardens, it will contract. Thus, the bump electrode 4 and the pad electrode 10 are kept pressed.
In such a conventional semiconductor device, as described above, by plastically deforming the layered portion of the bump electrode 4 and the pad electrode 10 and press-contacting them, an electric connection thereof is secured. The boding strength of the resin 11 causes the electric connection to be kept.
However, when the wiring board 5 comprises a resin, the wiring board would be bent, if the wiring board 5 is pressed or the temperature therearound varies due to its thermal expansion or contraction. Thus, tensions work at bonding interfaces between the semiconductor pellet 1 and the resin 11 and between the wiring board 5 and the resin 11. Consequently, the tensions cause the connecting surface between the bump electrode 4 and the pad electrode 10 to peel off.
When a crack occurs in the resin 11 disposed between the semiconductor pellet 1 and the wiring board 5, the pressuring state cannot be kept between the bump electrode 4 and the pad electrode 10, thereby deteriorating the electric connection.
An attempt for solving such a problem has been proposed in Japanese Patent Laid-Open Publication No. 6-268016. In this related art reference, as shown in FIG. 2, a groove or a concave portion 10a is formed at a pad electrode 10 of a wiring board 5. A bump electrode 4 is fitted to the concave portion 10a. In other words, the concave portion 10a allows a semiconductor pellet 1 and the wiring board 5 to align. Thus, the bump electrode 4 and the pad electrode 10 are connected without a displacement. As with the structure shown in FIG. 1, the semiconductor pellet 1 and the wiring board 5 are bonded with a resin. In the related art reference shown in FIG. 2, the bump electrode 4 comprises a solder. With a re-flowing process of the solder of the bump electrode 4, the bump electrode 4 and the pad electrode 10 are connected.
In the semiconductor device shown in FIG. 2, since an almost vertical side wall of the bump electrode 4 is surrounded by a side wall of the concave portion 10a of the pad electrode 10, even if the resin cracks, it is supposed that an electric connection between the bump electrode 4 and the pad electrode 10 is secured. However, when the wiring board 10 thermally expands or contracts in the direction in parallel with the direction of the semiconductor pellet 1, a displacement takes place between the bump electrode 4 and the pad electrode 10 in the direction of the thermal expansion or contraction. Thus, the side wall of the bump electrode 4 presses the side wall of the concave portion 10 and thereby the concave portion 10a deforms in such a manner that the width thereof increases. Thus, in the structure of which the bump electrode 4 and the pad electrode 10 are press-contacted, if the resin cracks, the contact of the semiconductor pellet 1 and the wiring board 5 becomes imperfect and thereby the electric connection thereof becomes imperfect.
To remove an unstable factor of the electric connection, it is necessary to connect the bump electrode 4 and the pad electrode 10 by the re-flowing process. The size of the pad electrode 10 should be larger than that of the structure shown in FIG. 1 because of the necessity of the concave portion 10a for the bump electrode 4. Thus, since the interval of the adjacent pad electrodes 10 becomes narrow, the withstand voltage drops and thereby a short-circuit tends to occur. As a result, the structure shown in FIG. 2 cannot be applied to the semiconductor device shown in FIG. 1.